FIG. 1 shows a Double Clutch Transmission (DCT) 1 which is adapted for transmitting a torque from an engine E to a differential D to which two vehicle wheels are operatively connected. Double Clutch Transmission 1 comprises a double clutch 2 with a hollow shaft 3 and a central output shaft 4. The clutch housing 5 is connected to the engine crankshaft 6. Both clutches 2a, 2b of clutch 2 are controlled by hydraulic pistons (not shown) integrated into the clutch housing 5. The central and hollow shafts 4 and 3 are connected to two separate gear sets 7, 8 each with their own synchronizers (not shown). One shaft 4 drives gear set 7 which comprises the odd gear ratios while the other shaft 3 drives gear set 8 which comprises the even gear ratios. While one clutch 2a or 2b is active, meaning that torque from engine E is being transferred to the respective gear set 7,8, the other clutch 2b or 2a remains open, meaning that no torque is being transferred to the respective other gear set 2b, 2a. Accordingly, a new gear can be selected via the synchronizer actuators among the gears in the currently inactive gear set 2b, 2a. 
By alternately coupling the central and the hollow shaft—with the synchronizers having selected the correct gear—shifts can be performed.
FIG. 2 shows an example of a possible hydraulic schema for controlling a double clutch, comprising a pressure supply 9 followed by a proportional actuator 10, a non-proportional actuator 11, a pressure sensor 12 and a connection 13a and 13b for each of the hydraulic pistons used in the two clutches 2a and 2b. 
Conventional double clutch shifts are performed as powershifts, meaning there is no torque interruption during the shift. To perform this kind of powershifts the incoming clutch needs to take over torque while the active clutch still transfers torque.
On the graph of FIG. 3 showing a simplified torque characteristic of the clutch, one can see that the touch point of the clutch needs to be reached before the clutch can control torque.
To perform powershifts the active clutch can only reduce its torque capacity if the incoming clutch has already taken over this torque from the active clutch. The incoming clutch needs to perform a clutch preparation phase to be able to transfer torque. From drivers perspective the preparation phase is only recognized as a delay, while the torque handover phase only results in a very smooth change in acceleration. FIG. 4 shows such a conventional double clutch shift performed as powershift.
The target of conventional clutch preparation methods is to control the actuators of a clutch such that it is ready to transfer torque, being the clutch is brought close to its touch point. Bringing the clutch to its touch point can be achieved with various control methods like there are model based or deterministic approaches. As one can see in FIG. 5 the output of such a clutch preparation strategy can be to over actuate the clutch, being actuated more than needed for the touch point, for a time which is shorter than the expected time, or a time which a model predicts, to reach the touch point.
When the clutch is prepared, so the touch point is reached, the incoming clutch can start to take over torque from the active clutch. The torque capacity of the active clutch is reduced in a way that it is guaranteed that the complete engine torque is transferred through one of the 2 clutches 2a or 2b and that the active clutch is not longer transferring torque when the incoming clutch torque capacity has reached the engine torque.
It is therefore an object of the present invention to provide a method of controlling a double clutch transmission that increases the responsiveness of such a double clutch transmission by decreasing the total time of preparing the incoming clutch and taking over the torque from the active clutch.
In order to achieve this object, the invention provides a method as defined above, which uses the following steps: First, the incoming clutch is prepared so as to be ready for taking over torque transmission, wherein the preparation is achieved by over actuating the incoming clutch. Then, a feedback signal from the incoming clutch is provided which indicates that the incoming clutch is prepared and has taken over at least a part of the engine torque from the active clutch. Then, the active clutch is shut off in a non-proportional way based on receiving the feedback signal. This results in reduced delay when switching torque transmission from one of the clutches to the other clutch.
Preferably, a calibration step is used for determining the amount of over actuation. This helps in performing a quick yet smooth change from one clutch to the other.
According to an advantageous embodiment, the calibration step is performed as a function of the engine speed, the throttle pedal position and/or other, as these parameters greatly influence the change from one clutch to the other.
Preferably, the method includes deciding based on the feedback signal of the incoming clutch that the clutch is prepared and transferring the complete or at least a part of the engine torque.
The above object is further achieved with a clutch control system for controlling a double clutch, the system comprising a transmission control unit (TCU) and at least two non-proportional valves, a first of the non-proportional valves being associated with a clutch actuation device for a first clutch, and a second of the non-proportional valves being associated with a clutch actuation device for a second clutch.